Long-Range Hydrophobic Attraction Between Graphene and Water/Oil Interfaces

Long-range hydrophobic attractions between mesoscopic surfaces in water play an important role in many colloid and interface phenomena. Despite being studied by several approaches, the origin of these forces has yet to be adequately explained. While previous research has focused on solid/water/solid and solid/water/air scenarios, we investigated a solid/water/liquid situation to gain additional insight. We directly measured the long-range interactions between a solid and a hydrophobic liquid separated by water using force spectroscopy, where colloidal probes were coated with graphene oxide (GO) to interact with immobilized heptane droplets in water. We detected attractions with a range of ~0.5 {\mu}m that cannot be explained by standard Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. When the GO was reduced to rGO to become more hydrophobic, these forces increased in strength and ranged up to 1.2 {\mu}m. This suggests that the observed attractions result from long-range hydrophobic forces. Based on our results, we propose air bubbles attached to the colloidal probe and molecular rearrangement at the water/oil interface as possible origins of the observed interactions. This knowledge will be useful to understand and motivate the formation of emulsions using 2D materials and other amphiphilic/hydrophobic particles

Enhancing polyimide\u27s water barrier properties through addition of functionalized graphene oxide

Graphene oxide produced by Tour\u27s method (GO) and GO functionalized with 4-4\u27 oxydianiline (ODAGO) are incorporated at 0.01 to 0.10 weight percent (wt%) into a polyimide (PI) made from 3,3\u27-benzophe-nonetetracarboxylic dianhydride (BTDA) and 4-4\u27 oxydianiline (ODA). The performance properties of these two systems GO-PI and ODAGO-PI at extremely low GO concentrations are compared. ODAGO-PI nanocomposite\u27s performance properties are comparable to previous results citing concentrations 10 times higher and displayed significantly greater improvement than unfunctionalized GO-PI films. The 0.01 wt% ODAGO-PI film demonstrated a factor of ten decrease in water vapor permeability. The 0.10 wt% ODAGO-PI film displayed the maximum increase of 82% in Young\u27s modulus. The water vapor permeability results were fit to the Nielsen law. We found that the model yielded unphysically large aspect ratios for the 0.01 wt% ODAGO-PI, 100 times larger than the AFM-measured value. For the GO-PI, we observe less enhancement of the barrier properties. The large aspect ratio indicates tortuosity effects alone cannot explain the enhanced barrier properties. We propose that the improved barrier properties are also due to a stabilizing effect of the flakes on the polymer matrix, where reduced mobility of the PI chain reduces diffusion through the polymer matrix. ATR-FTIR, WAXS, Raman and T-g results support this view. (C) 2016 Elsevier Ltd. All rights reserved

AFM-based mechanical characterization of single nanofibres

Nanofibres are found in a broad variety of hierarchical biological systems as fundamental structural units, and nanofibrillar components are playing an increasing role in the development of advanced functional materials. Accurate determination of the mechanical properties of single nanofibres is thus of great interest, yet measurement of these properties is challenging due to the intricate specimen handling and the exceptional force and deformation resolution that is required. The atomic force microscope (AFM) has emerged as an effective, reliable tool in the investigation of nanofibrillar mechanics, with the three most popular approaches—AFM-based tensile testing, three-point deformation testing, and nanoindentation—proving preferable to conventional tensile testing in many (but not all) cases. Here, we review the capabilities and limitations of each of these methods and give a comprehensive overview of the recent advances in this field

Surface oxide net charge of a titanium alloy Comparison between effects of treatment with heat or radiofrequency plasma glow discharge

In the current study we have compared the effects of heat and radiofrequency plasma glow discharge (RFGD) treatment of a Ti6Al4V alloy on the physico chemical properties of the alloys surface oxide Titanium alloy (Ti6Al4V) disks were passivated alone heated to 600 C or RFGD plasma treated in pure oxygen RFGD treatment did not alter the roughness topography elemental composition or thickness of the alloys surface oxide layer In contrast heat treatment altered oxide topography by creating a pattern of oxide elevations approximately 50-100 nm in diameter These nanostructures exhibited a three fold increase in roughness compared to untreated surfaces when RMS roughness was calculated after applying a spatial high-pass filter with a 200 nm cutoff wavelength Heat treatment also produced a surface enrichment in aluminum and vanadium oxides Both RFGD and heat treatment produced similar increases in oxide wettability Atomic force microscopy (AFM) measurements of metal surface oxide net charge signified by a long-range force of attraction to or repulsion from a (negatively charged) silicon nitride AFM probe were also obtained for all three experimental groups Force measurements showed that the RFGD treated Ti6Al4V samples demonstrated a higher net positive surface charge at pH values below 6 and a higher net negative surface charge at physiological pH (pH values between 7 and 8) compared to control and heat treated samples These findings suggest that RFGD treatment of metallic implant materials can be used to study the role of negatively charged surface oxide functional groups in protein bioactivity osteogenic cell behavior and osseointegration independently of oxide topography Published by Elsevier B

Graphene Oxide Reduces the Hydrolytic Degradation in Polyamide-11

Graphene oxide (GO) was incorporated into polyamide-11 (PA11) via in-situ polymerization. The GO-PA11 nano-composite had elevated resistance to hydrolytic degradation. At a loading of 1 mg/g, GO to PA11, the accelerated aging equilibrium molecular weight of GO-PA11 was higher (33 and 34 kg/mol at 100 and 120 C, respectively) compared to neat PA11 (23 and 24 kg/mol at 100 and 120 C, respectively). Neat PA11 had hydrolysis rate constants (kH) of 2.8 and 12 ( 10(exp -2) day(exp -1)) when aged at 100 and 120 C, respectively, and re-polymerization rate constants (kP) of 5.0 and 23 ( 10(exp -5) day(exp -1)), respectively. The higher equilibrium molecular weight for GO-PA11 loaded at 1 mg/g was the result of a decreased kH, 1.8 and 4.5 ( 10(exp -2) day(exp -1)), and an increased kP, 10 and 17 ( 10(exp -5) day(exp -1)) compared with neat PA11 at 100 and 120 C, respectively. The decreased rate of degradation and resulting 40% increased equilibrium molecular weight of GO-PA11 was attributed to the highly asymmetric planar GO nano-sheets that inhibited the molecular mobility of water and the polymer chain. The crystallinity of the polymer matrix was similarly affected by a reduction in chain mobility during annealing due to the GO nanoparticles' chemistry and highly asymmetric nano-planar sheet structure

Charge-Driven Selective Adsorption of Sodium Dodecyl Sulfate on Graphene Oxide Visualized by Atomic Force Microscopy

Using liquid-cell atomic force microscopy, we investigated molecular adsorption of sodium dodecyl sulfate (SDS) from aqueous solution onto single-layer graphenes that were oxidized to different degrees. SDS did not adsorb onto graphene oxide (GO) featuring an atomic carbon:oxygen (C:O) ratio of 2.0. Reduced GO (C:O ratio: 10.7), in contrast, was covered with micellar SDS aggregates, featuring a height of ≈2 nm and lateral feature sizes of ≈5 nm. This selective adsorption depending on the degree of oxidation can be explained by electrostatic repulsion between the negatively charged SDS head groups and the negatively charged hydroxyl groups on oxidized graphene. Our results suggest it will be possible to translate this adsorption selectivity into separation techniques that fractionate GO into subsets featuring different degrees of oxidation. Such fractionation techniques are likely to enable the preparation of GO with more well-defined electronic properties and make GO interesting as a band gap material

Tuning of structural color using a dielectric actuator and multifunctional compliant electrodes

We have developed electrically conducting silicone elastomer nanocomposites that serve both as compliant electrodes in an electrostatic actuator and, at the same time, as optically active elements creating structural color. We demonstrate the capabilities of our setup by actuating an elastomeric diffraction grating and colloidal-crystal-based photonic structures

Silk Reconstitution Disrupts Fibroin Self-Assembly

Using atomic force microscopy, we present the first molecular-scale comparison of two of the most important silk dopes, native (NSF) and reconstituted (RSF) silkworm fibroin. We found that both systems depended on shear to show self-assembly. Significant differences in the nature of self-assembly between NSF and RSF were shown. In the highest studied concentration of 1000 mg/L, NSF exhibited assembly into 20–30 nm-wide nanofibrils closely resembling the surface structures found in natural silk fibers. RSF, in contrast, showed no self-assembly whatsoever at the same concentration, which suggests that the reconstitution process significantly disrupts silk’s inherent self-assembly capability. At lower concentrations, both RSF and NSF formed fibrils under shear, apparently denatured by the substrate. Using image analysis, we quantified the properties of these self-assembled fibrils as a function of concentration and found low-concentration fibrils of NSF to form larger continuous structures than those of RSF, further supporting NSF’s superior self-assembly capabilities

Atomic Force Spectroscopy Using Colloidal Tips Functionalized with Dried Crude Oil: A Versatile Tool to Investigate Oil–Mineral Interactions

The recovery efficiency of water flooding (water injection in oil reservoirs to recover oil) can be improved by modification of the ionic composition of the brine. This effect is attributed to changes in the physicochemical interactions within in the crude oil–brine–rock (COBR) system. To systematically modify these interactions, gain predictive capability, and optimize recovery efficiency, further understanding of these systems is required. Our work introduces a new tool to facilitate the study of interactions in such ternary systems. Utilizing atomic force spectroscopy, we developed a custom dried oil-coated probe which directly measures the interactions between crude oil and a mineral substrate in a varying aqueous environment at their natural length scale. In most of the previous studies, COBR interactions were studied by using model systems wherein crude oil was represented by organic acids, for instance, which is a significant simplification of natural systems that may be unwarranted. In this study, we measured the interaction forces between mineral surfaces and actual crude oils. The experiments allow us to systematically test the effect of brine composition on the forces between mineral surfaces and crude oil components in all their complexity under realistic reservoir conditions. Our results illustrate the reproducibility of measurements made using this custom tool by using multiple probes to show salinity-dependent repulsive interactions between the oil coating and a mica substrate. These electrostatic interactions are consistent with trends expected by the Debye–Hückel theory, showing a decrease in repulsive forces as a function of increasing monovalent ion concentration. Adherence to this expected trend provides insight into the COBR interactions in a particular oil reservoir with particular oil, brine, and mineral compositions. Additionally, the presence of electrostatic forces suggests that the dried crude oil coating retains surface charge throughout the drying procedure. Therefore, this tool has the potential to represent or approximate the electrostatic interactions of the original liquid crude oil system during the study of COBR interactions